Optical routing in wavelength-division multiplexing optical systems requires a wavelength and polarization insensitive photonic switching fabric which includes a switch array. The switch array may be implemented in InGaAsP/InP optical devices using digital optical switches or laser-amplifier gate switches. An advantage of the digital optical switch approach is the low wavelength and polarization sensitivity of the digital optical switch. InP-based digital optical switches are typically more compact than those implemented in LiNbO.sub.3, and are also more easily integrated with other active devices to form large switch arrays. However, it is very difficult to achieve consistently good polarization-independent crosstalk of about -15 dB or less in a single Y-branch digital optical switch. This difficulty arises from the need to suppress second-order local normal mode excitation along the Y-branch structure. A conventional approach to reducing the local normal mode excitation is to allow for reasonably adiabatic modal evolution by utilizing relatively long devices of about 5 mm or more with small opening angles. Unfortunately, this approach increases propagation and bending losses and therefore leads to excessive switch loss.
Attempts to reduce the switch size and the associated excessive loss in the Y-branch digital optical switch have led to a number of "shaping" designs. Exemplary shaping designs are described in H. Okayama and M. Kawahara, "Reduction of Voltage-Length Product for Y-branch Digital Optical Switch," Journal of Lightwave Technology, Vol. 11, No. 2, pp. 379-387, February, 1993; W. K. Burns, "Shaping the Digital Switch," IEEE Photonics Technology Letters, Vol. 4, No. 8, pp. 861-863, August, 1992; and M. N. Khan et al., "Design and Demonstration of Weighted-Coupling Digital Y-branch Optical Switches in InGaAs/InGaAlAs Electron Transfer Waveguides," Journal of Lightwave Technology, Vol. 12, No. 11, pp. 2032-2039, November, 1994, all of which are incorporated by reference herein. These and other shaping designs often introduce coupling between the first order local normal mode and the second order mode, thereby substantially increasing the polarization and voltage sensitivity of the crosstalk. Moreover, the switch becomes more sensitive to fabrication tolerances and defects which tend to excite the second order mode, thereby further contributing to crosstalk.
As a result, dilated switching techniques are typically employed in applications requiring crosstalk values of about -25 dB or less. Dilating generally involves adding one or more additional switch stages to a given switch to further isolate the crosstalk. Exemplary dilated switching techniques are described in K. Padmanabhan and A. N. Netravali, "Dilated Networks for Photonic Switching," IEEE Transactions on Communications, Vol. COM-35, No. 12, pp. 1357-1365, December, 1987, which is incorporated by reference herein. However, additional switch stages not only increase the overall switch loss but also substantially increase the size of the switch array. Moreover, dilated switch arrays require more complex electronic control than comparable non-dilated switch arrays, in that the decision as to which switch elements of the dilated switching stage should be turned on or off depends on the specific signal path through the switch array.
The above-noted laser-amplifier gate switch approach provides inherently better crosstalk than the digital optical switch approach, typically on the order of -30 dB or less. However, the laser-amplifier gate switch approach leads to an accumulation of amplified spontaneous emission (ASE) noise in the switch array which can contribute to crosstalk. In addition, it is very difficult to achieve uniform performance and low polarization dependent loss (PDL) and crosstalk across a given chip and within the switch array. Moreover, the amplifier performance can be affected by the residual reflectivity at the interface between the active and passive regions, and the uniformity of that interface across the chip. These and other factors make the laser-amplifier gate switch approach difficult to implement in many practical optical switching applications. It would therefore be desirable to improve the crosstalk performance of the digital optical switch approach in a manner which avoids the problems associated with the conventional dilation or shaping approaches.
As is apparent from the above, a need exists for an improved digital optical switch which exhibits reduced crosstalk and can be implemented without the use of dilation or any other significant increase in switch size or switch array control complexity.